Surface mount soldering is the process of creating circuits using surface mount technology (SMT) components that are mounted directly onto the surface of printed circuit boards (PCBs).

SMT technology is a key driver in the ever-important work of miniaturization of circuits, allowing for smaller, lighter devices with ever-increasing capabilities.

Today’s cell phones have exponentially more computing power than the massive supercomputers that controlled the Apollo space missions.  The manufacture of PCB circuits via surface mount soldering was a key breakthrough that makes this possible.

Process

A pre-printed PCB outlining the circuit, with solder pads indicating component locations and connections, is matched with a stencil that directs the application of solder paste.  

Solder paste is a gelatinous mixture of powdered conductive metal (commonly tin) and flux, which is a liquid or semi-liquid chemical compound that acts as a binding, cleaning, and wetting agent.

The solder paste is commonly spread in a wide swatch over the stencil, which directs it to only those solder pads where components are to be attached and allows for easy reclamation of excess paste for re-use.

Once the paste is applied, a conveyor belt runs the PCB through a pick-and-place machine where components are simply set onto the board.  Tolerances for placement are generally relatively more forgiving than other solder methods, as the coming reflow process will automatically align the components on their pads.

When both paste and components are in place, the PCB then transits through a reflow oven, which heats the board at a rate designed to melt the solder paste without damaging the components. The reflow process liquefies the metal component of the solder paste, cementing the contact points of the electrical components to the PCB and ensuring electrical conductivity, while common evaporating the excess flux.  The surface tension of the molten solder helps keep the components in place.

Reflow ovens for surface mount soldering typically come in one of three categories: convection, infrared, or vapor phase

Following reflow, a washing of the completed board is recommended.  This both removes any stray solder balls that can cause short circuits, as well as remove flux residues that may have spread onto the PCB during the heating process. 

Depending on the type of solder paste used, different solvents are required:

• No-clean fluxes are most common, and they do not require strict washing.  The flux residue is considered harmless, and can simply be left on the board.  However, a manufacturing process that skips the washing step requires a very clean assembly environment and may need an inert reflow environment to prevent conductive particles from landing on the board.  If a conformal coating is to be applied, washing is still recommended, as the flux residue could cause those coatings to adhere less effectively.
• Water-soluble fluxes can be removed with a directed water spray, typically including a detergent compound
• Rosin-based fluxes require specific chemical solvents, generally involving fluorocarbons

Advantages

Because surface mount soldering allows for machine-guided placement of components and printed stencil-guided application of solder, a much more tightly packed circuit board is possible.  The advantages here are numerous, including:

• Smaller components (from one-quarter to one-tenth the size and weight)
• Higher component density
• More connections per component
• Lower radiated emissions (due to the smaller radiation loop area and inductance inherent to smaller components)
• Lower resistance and inductance at the connection
• Cheaper components (again, generally due to smaller components – from one-half to one-quarter the cost of equivalent through-hole parts)
• Final products that both take up less space and have less mass

Surface mount soldering has also proven to be the most efficient and cost-effective process for mass-production of printed circuit boards.  Because the setup is almost entirely automated equipment, there is less time, setup and labor cost involved, as well as simpler and faster operation once the process is in place. On the high-end, some pick-and-place machines are capable of placing more than 2,000 components per minute.

As mentioned previously, the reflow step key to the surface mount soldering process allows small errors in component placement to be corrected automatically as the surface tension of molten solder pulls components into alignment with solder pads.

Disadvantages

Because surface mount soldering relies on the surface tension of the melted solder paste to hold components to the board, it is not recommended for large components, nor those with high-voltage capacities.  In PCBs that use transformers, heat-sinked power semiconductors, large capacitors, or fuses, either additional adhesive and/or a combination of both surface mount and through-hole soldering is used.

Through-hole soldering also adds a degree of mechanical connection between components and boards by extending leads through PCBs, contrasted to the simple seated SMT model.  This means surface mount soldering creates a mechanically weaker connection and is not preferred for components that will be undergoing sheer force.

The key advantage of surface mount soldering, with the automation process allowing smaller and more dense packing of components, also means that repairs can be difficult.  

The higher speed and miniaturization in the production process can increase the risk of defects, so a failure detection process is recommended for any SMT manufacturing process. Surface mount soldering is prone to a variety of potential issues and these issues can begin with solder paste printing.  

FCT Assembly and our partner BlueRing Stencils offer a variety of high-quality products which can improve the solder paste printing process.  

Our solder pastes have excellent printability and long stencil life. Our stencils and nano-coatings are designed to improve the print process.  

FCT Assembly offers technical support for the entire surface mount soldering process.  Please contact us if you need help with surface mount soldering.

Solder paste flux is a gelatinous chemical compound used mainly in combination with solder powder to create solder paste, generally mixed as a 50/50 ratio.  Once combined, the paste is a gray, putty-like material.

Solder paste flux serves a threefold purpose:

• Removing any oxidized metal from the surfaces to be soldered
• Sealing out air, preventing further oxidation
• Improving wetting characteristics of liquid solder by facilitating amalgamation.

Chemical Properties

Solder paste flux has a very complex chemical composition.  It can be comprised of:

• Solvents – liquid used an inert medium to combine the active ingredients, such as water
• Rosins/Resins – high-temperature tolerant, non-volatile chemicals, most commonly forms of glycol
• Surfactants – a detergent used to lower the contact angle of the flux with the surface its applied to, allowing greater coverage
• Activators – chemicals disrupting/dissolving metal oxides, typically acids of various corrosivity
• Rheology Modifiers – any of a variety of compounds that allow a liquid to display Non-Newtonian properties; that is, to change viscosity as pressure is varied

Flux is nearly inert at room temperature but becomes strongly reducing when its temperature is increased.  This prevents the formation of metal oxides on both the base and filler materials to which it is applied.  It also prevents the beading of the applied solder, facilitating easy flows and application.

Solder paste flux is different than liquid flux in both physical form and activity level.  Solder paste flux typically contains 60-80% by weight active ingredients, compared with liquid flux’s 2-25%.  

By weight, a mixed solder paste commonly consists of 90% metal.  However, the non-metallic ingredients of the flux, due to their lower density, result in their comprising of nearly half the paste volume.

Standards and Definitions

A number of standards exist to define the various flux types. The principal standard is J-STD-004, which categorize fluxes by composition, activity (strength), the presence or lack of halide activators, and reliability from electromigration standpoint and surface insulation resistance (SIR). The J-STD-004 standard for electromigration and surface insulation resistance is >100 MΩ after one week at elevated temperature and humidity with a DC bias applied.

There are three types of solder paste flux:

Use

While the main use of solder paste flux is in the creation of solder paste, it is also sold directly as a gel or tacky flux.  

Solder paste flux can also act as a temporary adhesive.  Its tacky nature holds components together until the heat of the soldering process melts the solder and the parts are fused together.  

Once mixed, solder paste is most commonly used in a stencil-printing process. Paste is spread over a stencil in the pattern required on the final end product, a printed circuit board.

The solder paste flux component must have enough tackiness to secure the components while the fixture is processed through the manufacturing line.

Once printed, the circuit board forming process is followed by pre-heating and reflow (melting).

For the flux, the temperature and rate of change in temperature are key in the reflow process.  A slow rise in temperature is necessary to prevent solder balling, but must be great enough to activate the flux, then melt the solder itself, and recool quickly enough to retain the desired shape.

Application

Solder paste flux gel can be brushed on where needed and, unlike liquid flux, will not flow away from the application area.  

This means that, when used directly as a soldering aid, a small amount of solder paste flux can be used in place of a relatively large amount of liquid flux. Common uses of flux gel include BGA re-balling and the rework/repair of solder joints.

In mixing solder paste flux and solder powder for application to circuit assemblies, there are important considerations:

Working Life

Different types of flux, particularly water-based, can gradually evaporate after mixing, eventually hardening to the point of unusability.  Pre-mixed solder pastes have this measurement in time provided by the manufacturer.  The working life of self-mixed pastes can be approximated, but should be underestimated for safety.

Viscosity

As a liquid, solder paste is not entirely immune to flow.  The thickness of the flux/powder mixture helps determine how well it resists this tendency to hold its shape through the curing process.  However, sometimes a lower viscosity is preferred.  

When using a squeegee to apply solder paste onto a stencil, the force used to apply can break down the viscosity, which thins the paste and allows it to more easily flow through the stencil apertures.  Again, pre-mixed pastes have manufacturer-provided measurements, while custom mixes often require self-testing.

Both working life and viscosity can be expressed through the thixotropic index, a measurement of a mixed solder paste’s viscosity at rest versus applied.

Depending on the ratio and composition of the solder paste flux and solder powder, stirring may be needed at frequent intervals to ensure proper viscosity and test for working life.

The amount of paste is also a key consideration.  Too little may cause a joint to lack strength or an electrical connection to have poor conductivity.  Too much can result in accidental short-circuits on the board.  

Storage & Cleanup

Solder paste flux needs proper storage to maintain usability.  An airtight container must be used to prevent evaporation.  Storage in an area of a low temperature reduces the rate of oxidation, and thus the rate of degradation of the flux – but take care not to introduce it to freezing temperatures, which can cause separation of the chemicals.

As with all fluxes used in electronics, residues left behind may be harmful to the circuit, and standards exist to measure the safety of the residues left behind.

Each type of flux requires its own solvents for cleanup of excess:

The residual flux is considered harmless, and can simply be left to dry on the board.  Their use necessitates a very clean assembly environment and may need an inert reflow environment.  

Washing is still recommended, however, as residual no-clean flux can cause conformal coatings to adhere less effectively.

FCT Assembly manufactures and sells a wide variety of soldering products. These include solder pastes, wire solder, bar solder, liquid flux, and gel flux. 

Contact us for more details!